1. Field of the Invention
The present invention relates to a plasma display panel (hereinafter also referred to as a PDP) used for a flat-type TV set and others and a plasma display device employing the plasma display panel, and in particular to a structure of a plasma display panel capable of realizing the improvement of its display brightness and display contrast.
2. Description of Prior Art
The plasma display panel is used in a large-screen, small-depth, flat-screen TV set, and has improved in performance. However, its light-room display contrast, that is, a contrast as measured in a well-lighted environment (usually assumed to be a living room provided with an ambient room illumination producing 150-200 l×), is not satisfactory yet.
FIG. 2 is an exploded perspective view of part of a structure of an example of a typical plasma display panel. The plasma display panel has a structure in which front and rear substrates are attached together and a discharge gas is filled therebetween.
The front substrate includes a plurality of electrode pairs each comprised of a transparent electrode 2 and a bus electrode 3 for producing a sustain discharge (also called a display discharge) disposed on a front glass plate 1 (usually, one electrode of the electrode pair is called an X electrode, and the other electrode of the electrode pair is called a Y electrode. In FIG. 2, only one pair of the plural electrode pairs is shown. The electrode pairs are covered with a dielectric 4 and a protective film 5.
The rear substrate includes address electrodes 9 disposed on a rear glass plate 6, and the address electrodes 9 are covered with a dielectric 8. Barrier ribs 7 are disposed on the dielectric 8, and red, blue and green phosphor films 10 are disposed between the barrier ribs 7, respectively.
The front and rear substrates are aligned with each other and are sealed together such that the electrodes on the front substrate intersect those on the rear substrate at approximately right angles (in some cases, such that the electrodes on the front substrate intersect those on the rear substrate at angles other than the approximately right angles). A space between the two substrates is filled with a discharge gas, and thereby a plurality of cells are formed. A discharge is created in a desired one of the plurality of cells, by selectively applying appropriate voltages to the sustain electrode pairs on the front substrate and the address electrodes on the rear substrate. By this main discharge, vacuum ultraviolet light is produced, emission of red, blue and green lights is generated from the respective ones of the red, blue and green phosphor films 10 excited by the produced vacuum ultraviolet light, thereby producing a full-color display.
However, since the body color of the phosphor 10 is usually close to white, ambient light incident on the plasma display panel is reflected by the phosphor film 10, and degrades the display contrast.
Japanese Patent Application Laid-Open No. 2004-31287 Publication discloses a method of improving display contrast which realizes higher display contrast by suppressing degradation of display brightness using a striped laminated member composed of a light absorption layer and a light reflection layer. FIG. 3 is a front view of a plasma display panel of an example disclosed in this publication, and FIG. 4 is a cross-sectional view of the plasma display panel of FIG. 3 taken along line IV-IV′ of FIG. 3. The laminated member 130 is composed of a light absorption layer 110 and a light reflection layer 120, and ambient light incident on the plasma display panel is absorbed by the light absorption layer 110. On the other hand, light which is incident onto the light reflection layer 120 from a phosphor film 10 is reflected back toward the phosphor film 10, then is reflected again by the phosphor film 10, and then is emitted into the outside of the plasma display panel.
FIG. 5 illustrates a phenomenon which happens in a case where an aperture ratio of a discharge cell is reduced so as to realize a higher display contrast ratio by using the above conventional technique. Light from the phosphor film 10 at the peripheral portions of one discharge cell undergoes multiple reflections between the phosphor film 10 and the light reflection layers 120. If light reflections on the surface of one of or the surfaces of both the phosphor film 10 and the light reflection layers 120 are diffuse reflections, the number of the multiple reflections increases even more. In this case, since the reflectance of the phosphor film 10 and the light reflection layers 120 is less than 100%, no small amount of the light is absorbed. Consequently, the intensity of the light emitted from the plasma display panel is reduced as the number of light reflections is increased within the discharge cells. Therefore, as the aperture ratio is reduced for the purpose of improving the display contrast in the above conventional technique, the display brightness is reduced.
Although the device has been described in connection with the so-called ac surface-discharge three-electrode type PDP, it is needless to say that the present invention is applicable to various types of PDPs. For example, the present invention is applicable to dc-type PDPs as disclosed in Mikoshiba, S: “Up-to-date Technology for Plasma Displays,” chap. 6, ED Research Company, Tokyo, 1996, and is also applicable to vertical-discharge type PDPs as disclosed in G. Baret, et al.: 14.4: A 640×480 High-Resolution Color ac Plasma Display, SID 93 DIGEST, pp. 173-175.
In connection with the PDP of the above-explained structure, a full-color display has been explained as formed by exciting the respective primary-color phosphors to emit red, blue and green light with vacuum ultraviolet light produced by the main discharge. However, needless to say, the present invention is not only applicable in a case where the phosphors are excited by vacuum ultraviolet light, but is also applicable in a case where the phosphors are excited by ultraviolet light other than the vacuum ultraviolet light. Further, needless to say, while the PDP of the above-explained structure generates visible lights of red, blue and green by using the phosphors, the present invention is also applicable to PDPs of a structure capable of generating visible lights directly by discharges. Further, needless to say, the present invention is also applicable in a case where visible lights of colors other than red, blue and green are generated, and in a case where a visible light of a single color is generated.